I haven't looked into HelloSign or its pricing model, but I'll say that, so far, HelloFax seems to have handled its sort of "freemium" features and promotions, and its pricing model fairly well. (Without my having access to any of their financials or management, but having used the product and observed its continued existence and some news about same, over the past year and a bit).
They seem to be good at getting you on board and then upselling you based upon your actual needs and use, rather than coercive arm twisting.
I'm more concerned about installing any plugin and giving it access to my Gmail account. Buchheit's participation is a coup, in that regard, for me and I suspect for some others aware of his role in Gmail's original development, at least.
And that same crowd seems to form part of the original core of their paying business. (Again, this is a fairly uninformed observation, on my part.)
I'll be curious to observe where this goes.